Open magnet with shielding

ABSTRACT

An open magnet useful in magnetic resonance imaging (MRI) applications. The magnet has two spaced-apart assemblies. Each assembly has a shielding coil located longitudinally outward from a main coil and a magnetizable pole piece spaced apart from and proximate the main and shielding coils. The method of the invention generates a magnetic field in a first area between the two assemblies while shielding a second area not between the two assemblies from a stray magnetic field by creating the previously-described open magnet.

CROSS-REFERENCE RELATED APPLICATIONS

The present patent application is a continuation-in-part application ofU.S. patent application Ser. No. 09/035,639 by Evangelos T. Laskaris etal. which is entitled "Shielded And Open Superconductive Magnet" andwhich was filed Mar. 5, 1998 (and which issued as U.S. Pat. No.5,874,880 on Feb. 23, 1999).

FIELD OF THE INVENTION

The present invention relates generally to an open magnet used togenerate a uniform magnetic field, and more particularly to such amagnet having shielding to protect the area around the magnet from straymagnetic fields originating from the magnet.

BACKGROUND OF THE INVENTION

Magnets include resistive and superconductive magnets which are part ofa magnetic resonance imaging (MRI) system used in various applicationssuch as medical diagnostics. Known superconductive magnets includeliquid-helium-cooled and cryocooler-cooled superconductive magnets.Typically, the superconductive coil assembly includes a superconductivemain coil surrounded by a first thermal shield surrounded by a vacuumenclosure. A cryocooler-cooled magnet typically also includes acryocooler coldhead externally mounted to the vacuum enclosure, havingits first cold stage in thermal contact with the thermal shield, andhaving its second cold stage in thermal contact with the superconductivemain coil. A liquid-helium-cooled magnet typically also includes aliquid-helium vessel surrounding the superconductive main coil and asecond thermal shield which surrounds the first thermal shield whichsurrounds the liquid-helium vessel.

Known resistive and superconductive magnet designs include closedmagnets and open magnets. Closed magnets typically have a single,tubular-shaped resistive or superconductive coil assembly having a bore.The coil assembly includes several radially-aligned and longitudinallyspaced-apart resistive or superconductive main coils each carrying alarge, identical electric current in the same direction. The main coilsare thus designed to create a magnetic field of high uniformity within atypically spherical imaging volume centered within the magnet's borewhere the object to be imaged is placed. A single, tubular-shapedshielding assembly may also be used to prevent the high magnetic fieldcreated by and surrounding the main coils from adversely interactingwith electronic equipment in the vicinity of the magnet. Such shieldingassembly includes several radially-aligned and longitudinallyspaced-apart resistive or superconductive shielding coils carryingelectric currents of generally equal amperage, but in an oppositedirection, to the electric current carried in the main coils andpositioned radially outward of the main coils.

Open magnets, including "C" shape magnets, typically employ twospaced-apart coil assemblies with the space between the assembliescontaining the imaging volume and allowing for access by medicalpersonnel for surgery or other medical procedures during magneticresonance imaging. The patient may be positioned in that space or alsoin the bore of the toroidal-shaped coil assemblies. The open space helpsthe patient overcome any feelings of claustrophobia that may beexperienced in a closed magnet design. Known open magnet designs havingshielding include those wherein each coil assembly has an open bore andcontains a resistive or superconductive shielding coil positionedlongitudinally and radially outward from the resistive orsuperconductive main coil(s). In the case of a superconductive magnet, alarge amount of expensive superconductor is needed in the main coil toovercome the magnetic field subtracting effects of the shielding coil.Calculations show that for a 0.75 Tesla magnet, generally 2,300 poundsof superconductor are needed yielding an expensive magnet weighinggenerally 12,000 pounds. The modest weight makes this a viable magnetdesign.

It is also known in open magnet designs to place an iron pole piece inthe bore of a resistive or superconductive coil assembly which lacks ashielding coil. The iron pole piece enhances the strength of themagnetic field and, by shaping the surface of the pole piece,magnetically shims the magnet improving the homogeneity of the magneticfield. An iron return path is used to connect the two iron pole pieces.It is noted that the iron pole piece also acts to shield the magnet.However, a large amount of iron is needed in the iron pole piece toachieve shielding in strong magnets. In the case of a superconductivemagnet, calculations show that for a 0.75 Tesla magnet, only generally200 pounds of superconductor are needed yielding a magnet weighing over70,000 pounds which is too heavy to be used in medical facilities suchas hospitals. The weight does not make this a viable magnet design.

What is needed is an open magnet design having shielding which is lightenough to be used in medical facilities and which is less expensive thanknown designs.

SUMMARY OF THE INVENTION

In a first embodiment, the open magnet of the invention includes a firstassembly and a second assembly longitudinally spaced apart from thefirst assembly. The first assembly includes a generallylongitudinally-extending first axis, at least one main coil, at leastone shielding coil, and at least one magnetizable pole piece. The atleast one main coil, the at least one shielding coil, and the at leastone magnetizable pole piece are generally coaxially aligned with thefirst axis. The at least one main coil carries a first main electriccurrent in a first direction, and the at least one shielding coilcarries a first shielding electric current in a direction opposite tothe first direction. The at least one shielding coil is positionedlongitudinally outward from the at least one main coil. The at least onemagnetizable pole piece is spaced apart from and proximate the at leastone main and shielding coils.

The second assembly includes a generally longitudinally-extending secondaxis which is generally coaxially aligned with the first axis, at leastone main coil, at least one shielding coil, and at least onemagnetizable pole piece. The at least one main coil, the at least oneshielding coil, and the at least one magnetizable pole piece aregenerally coaxially aligned with the second axis. The at least one maincoil carries a first main electric current in the previously-describedfirst direction, and the at least one shielding coil carries a firstshielding electric current in the previously-described oppositedirection. The at least one shielding coil is positioned longitudinallyoutward from the at least one main coil. The at least one magnetizablepole piece is spaced apart from and proximate the at least one main andshielding coils.

In a first example, the method of the invention, for generating amagnetic field in a first area between a first and second assembly whileshielding a second area not between said first and second assembly froma stray magnetic field emanating from the generation of the magneticfield in said first area, includes several steps. A step includespositioning as a first assembly at least one main coil, at least oneshielding coil, and at least one magnetizable pole piece, wherein the atleast one main coil carries a first electric current in a firstdirection, wherein the at least one shielding coil is locatedlongitudinally outward from the at least one main coil and carries afirst shielding electric current in a direction opposite to the firstdirection, and wherein the at least one magnetizable pole piece isspaced apart from and proximate the at least one main and shieldingcoils. Another step includes positioning as a second assembly at leastone main coil, at least one shielding coil, and at least onemagnetizable pole piece, wherein the at least one main coil carries asecond electric current in the previously-described first direction,wherein the at least one shielding coil is located longitudinallyoutward from the at least one main coil and carries a second shieldingelectric current in the previously-described opposite direction, andwherein the at least one magnetizable pole piece is spaced apart fromthe at least one main and shielding coils. An additional step isgenerally coaxially aligning the at least one main and shielding coilsand pole piece of the first and second assemblies with a generallylongitudinally-extending axis. A further step is longitudinally spacingapart the second assembly from the first assembly.

Several benefits and advantages are derived from the invention. The atleast one pole piece enhances the strength of the magnetic field so lessconductor or superconductor is needed in the at least one main coil. Theportion of the at least one pole piece proximate the at least one maincoil provides a partial magnetic flux return for the at least one maincoil which reduces the magnetizable material needed in the at least onepole piece and which reduces the amount of conductor or superconductorneeded in the at least one main coil. The portion of the at least onepole piece proximate the at least one shielding coil also magneticallydecouples the at least one shielding coil from the at least one maincoil so that the magnetic flux lines from the at least one shieldingcoil are captured by the portion of the at least one pole pieceproximate the at least one shielding coil and do not reach the magneticflux lines from the at least one main coil. Thus the magnetizable massof the at least one pole piece does not have to be increased, and theamount of the conductor or superconductor in the at least one main coildoes not have to be increased, to offset the field subtracting effectsof the magnetic flux lines from the at least one shielding coil, sincethey are blocked by the presence of the portion of the at least one polepiece proximate the at least one shielding coil. In the case of asuperconductive magnet, computer simulations show that a 0.75 Teslamagnet of the present invention would use generally 750 pounds ofsuperconductor yielding a magnet weighing generally 15,000 pounds (whichis light enough to be installed in a medical facility) and costing onlyhalf or less of what a viable equivalent conventional magnet would cost.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic front elevational view of a first preferredembodiment of the open magnet of the invention;

FIG. 2 is a schematic top planar view of the magnet of FIG. 1;

FIG. 3 is a schematic cross sectional view of the magnet of FIGS. 1 and2 taken along lines 3--3 of FIG. 2 with the addition of a cryocoolercoldhead; and

FIG. 4 is a detailed schematic view of a portion of the magnet shown inFIG. 3 without the presence of the liquid cryogen.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, wherein like numerals represent likeelements throughout, FIGS. 1-4 show a first embodiment of the openmagnet 10 of the present invention. Magnet 10 is a superconductivemagnet. In an example, the magnet 10 is a 0.5 or higher Tesla magnet.The magnet 10 includes a first assembly 12. The first assembly 12includes a generally annular-shaped first superconductive main coil 14having a generally longitudinal first axis 16, having a longitudinallyinner end 18, and having a longitudinally outer end 20. The firstsuperconductive main coil 14 carries a first main electric current in afirst direction. The first direction is defined to be either a clockwiseor a counterclockwise circumferential direction about the first axis 16with any slight longitudinal component of current direction beingignored. It is noted that additional first superconductive main coilsmay be needed to achieve a high magnetic field strength, within themagnet's imaging volume, without exceeding the critical current densityof the superconductor being used in the superconductive coils, as isknown to those skilled in the art. An example of a superconductor forthe first superconductive main coil 14 is niobium-titanium.

The first assembly 12 also includes a generally annular-shaped firstsuperconductive shielding coil 22 generally coaxially aligned with thefirst axis 16. The first superconductive shielding coil 22 is spacedlongitudinally outward and apart from the first superconductive maincoil 14 and at least partially radially overlaps the firstsuperconductive main coil 14. For the purposes of describing theinvention, a first magnet component is said to "at least partiallyradially overlap" a second magnet component if the two components wouldcollide as they are moved together, with any intervening magnetcomponents removed, in a direction parallel to the longitudinal axis. Itis noted that a first component can completely radially overlap a secondcomponent only when the first component has a radial thickness which isequal to, or greater than, the radial thickness of the second component.The first superconductive shielding coil 22 carries a first shieldingelectric current in a direction opposite to the previously-defined firstdirection. An example of a superconductor for the first superconductiveshielding coil 22 is niobium-titanium.

The first assembly 12 additionally includes a magnetizable and generallycylindrical-shaped first pole piece 24 generally coaxially aligned withthe first axis 16 and spaced apart from the first superconductive mainand shielding coils 14 and 22. The first pole piece 24 has a firstradially-outer portion 26 at least partially radially overlapping thefirst superconductive main coil 14. The first pole piece 24 also has afirst longitudinally-inner portion 28 which has a generally annularshape and which is generally coaxially aligned with the first axis 16.The first longitudinally-inner portion 28 is disposed radially closer tothe first radially-outer portion 26 than to the first axis 16, and thefirst longitudinally-inner portion 28 projects longitudinally inwardpast the longitudinally outer end 20 of the first superconductive maincoil 14. In an example, the first pole piece 24 consists essentially ofa ferromagnetic material. In an example, the first pole piece 24consists essentially of iron.

The open superconductive magnet 10 also includes a second assembly 30.The second assembly 30 includes a generally annular-shaped secondsuperconductive main coil 32 longitudinally spaced apart from the firstsuperconductive main coil 14. The second superconductive main coil 32has a generally longitudinal second axis 34 generally coaxially alignedwith the first axis 16. The second superconductive main coil 32 also hasa longitudinally inner end 36 and a longitudinally outer end 38. Thelongitudinally inner ends 18 and 36 of the first and secondsuperconductive main coils 14 and 32 are longitudinally closer to eachother than are the longitudinally outer ends 20 and 38 of the first andsecond superconductive main coils 14 and 32. The second superconductivemain coil 32 carries a second main electric current in thepreviously-mentioned first direction. It is noted that additional secondsuperconductive main coils may be needed to balance any additional firstsuperconductive main coils present in the first assembly, as is known tothose skilled in the art. An example of a superconductor for the secondsuperconductive main coil 32 is niobium-titanium.

The second assembly 30 also includes a generally annular-shaped secondsuperconductive shielding coil 40 generally coaxially aligned with thesecond axis 34. The second superconductive shielding coil 40 is spacedlongitudinally outward and apart from the second superconductive maincoil 32 and at least partially radially overlaps the secondsuperconductive main coil 32. The second superconductive shielding coil40 carries a second shielding electric current in the previously-definedopposite direction. An example of a superconductor for the secondsuperconductive shielding coil 40 is niobium-titanium.

The second assembly 30 additionally includes a magnetizable andgenerally cylindrical-shaped second pole piece 42 longitudinally spacedapart from, and without a magnetizable solid path to, the first polepiece 24. The second pole piece 42 is generally coaxially aligned withthe second axis 34 and is spaced apart from the second superconductivemain and shielding coils 32 and 40. The second pole piece 42 has asecond radially-outer portion 44 at least partially radially overlappingthe second superconductive main coil 32. The second pole piece 42 alsohas a second longitudinally-inner portion 46 which has a generallyannular shape and which is generally coaxially aligned with the secondaxis 34. The second longitudinally-inner portion 46 is disposed radiallycloser to the second radially-outer portion 44 than to the second axis34, and the second longitudinally-inner portion 46 projectslongitudinally inward past the longitudinally outer end 38 of the secondsuperconductive main coil 32. In an example, the second pole piece 42consists essentially of a ferromagnetic material. In an example, thesecond pole piece 42 consists essentially of iron.

In an exemplary construction, the open superconductive magnet 10includes only one support member 48 connecting the first and secondassemblies 12 and 30, wherein the support member 48 is a nonmagnetizablesupport member preferably consisting essentially of nonmagneticstainless steel. In an example, the support member 48 and the first andsecond assemblies 12 and 30 together have a generally "C" shape whenviewed in a cross section created by a cutting plane, wherein the firstaxis 16 lies completely in the cutting plane, and wherein the cuttingplane generally bisects the support member 48. It is noted that thepreviously-defined cross section is the cross section shown in FIG. 3,with the "C" shape seen by rotating FIG. 3 ninety degreescounterclockwise.

In an example, the second assembly 30 is a general mirror image of thefirst assembly 12 about a plane 50 (seen on edge as a dashed line inFIG. 3) disposed longitudinally equidistant between the first and secondassemblies 12 and 30 and oriented generally perpendicular to the firstaxis 16. When the magnet 10 is employed as an MRI (magnetic resonanceimaging) magnet, the magnet 10 includes a magnetic resonance imagingvolume 52 (seen as a dotted line in FIGS. 1-3) disposed generallylongitudinally equidistant between the first and second assemblies 12and 30. In an example, the imaging volume is a generally sphericalimaging volume having a center 54 lying generally on the first axis 16.A patient 55 is shown in an imaging position in FIGS. 1 and 2. As isknown to the artisan, the magnet 10 and the patient 55 can be rotatedninety degrees clockwise from their positions shown in FIGS. 1 and 2,with the patient suitably supported on a patient table.

It is noted that the superconductive coils 14, 22, 32, and 40 arecooled, to a temperature below their critical temperature to achieve andsustain superconductivity, preferably by liquid-helium (or othercryogenic) cooling, by cryocooler cooling, or by a combination thereof.In a first cooling option, illustrated in the first assembly 12, themagnet 10 also includes a first cryogenic vessel 56 surrounding thefirst superconductive main and shielding coils 14 and 22, wherein thefirst pole piece 24 is disposed outside and spaced apart from the firstcryogenic vessel 56. The first cryogenic vessel 56 contains a liquidcryogen 58, such as liquid helium. In an example, the first cryogenicvessel 56 consists essentially of aluminum or nonmagnetic stainlesssteel. Here, the magnet 10 further includes a plurality of plates 60,62, 64, 66, and 68 which, together with the first pole piece 24 define afirst vacuum enclosure 70 which surrounds and which is spaced apart fromthe first cryogenic vessel 56. In an exemplary construction, the plates60, 62, 64, 66, and 68 consist essentially of nonmagnetic stainlesssteel.

In a second cooling option, illustrated in the second assembly 30, themagnet 10 also includes a cryocooler coldhead 72 having a housing 74attached to the second vacuum enclosure 76 and having a cold stage 78 insolid thermal conduction contact with the second superconductive mainand shielding coils 32 and 40. Other cooling options (not shown in thefigures) include each assembly having its own cryogenic vessel, whereinthe liquid cryogen in one cryogenic vessel is in fluid communicationwith the liquid cryogen in the other cryogenic vessel through aninterconnecting duct separate from the support member. Alternately, asolid thermal conduction path can be placed in the interconnecting ductallowing the cryocooler coldhead on the second vacuum enclosure to alsocool the superconductive main and shielding coils in the first vacuumenclosure.

In an exemplary embodiment, as seen in FIG. 3, the first radially-outerportion 26 is a radially-outermost portion 80 of the first pole piece24, and the first longitudinally-inner portion 28 is alongitudinally-innermost portion 82 of the first pole piece 24. In anexample, the first superconductive shielding coil 22 is spacedlongitudinally-outwardly apart from the first pole piece 24. In anexample, the first superconductive shielding coil 22 partially radiallyoverlap the radially-outermost portion 80 of the first pole piece 24 andgenerally completely radially overlap the first superconductive maincoil 14. In an exemplary construction, the longitudinally-innermostportion 82 of the first pole piece 24 projects longitudinally inwardpast the longitudinally inner end 18 of the first superconductive maincoil 14. In an example, the first superconductive main coil 14 isdisposed longitudinally closer than the first superconductive shieldingcoil 22 to the radially-outermost portion 80 of the first pole piece 24.In an exemplary construction, the magnet 10 of FIG. 3 is a generally0.75 Tesla magnet.

It is noted that those skilled in the art, using computer simulationsbased on conventional magnetic field analysis techniques, and using theteachings of the present invention, can design a shielded and openmagnet of a desired magnetic field strength, a desired level of magneticfield inhomogeneity, and a desired level of shielding (i.e., a desiredposition of the 5 Gauss stray field from the center of the imagingvolume of the open superconductive magnet). It is noted, as shown inFIG. 3, that such analysis shows that a coaxially-aligned disk of ironcan be removed from the longitudinally-outer area 84 of the first polepiece 24 without effecting the performance of the magnet 10, as can beappreciated by those skilled in the art. As previously mentioned, thepole piece enhances the strength of the magnetic field so lesssuperconductor is needed in the main coil. The radially-outermostportion of the pole piece provides a partial magnetic flux return forthe main coil which reduces the iron needed in the pole piece and whichreduces the amount of superconductor needed in the main coil. Theradially-outermost portion of the pole piece also magnetically decouplesthe shielding coil from the main coil so that the magnetic flux linesfrom the shielding coil are captured by the radially-outermost portionof the pole piece and do not reach the magnetic flux lines from the maincoil. Thus the iron mass of the pole piece does not have to beincreased, and the amount of the superconductor in the main coil doesnot have to be increased, to offset the field subtracting effects of themagnetic flux lines from the shielding coil, since they are blocked bythe presence of the radially-outermost portion of the pole piece.Computer simulations show that a 0.75 Tesla magnet of the presentinvention would use generally 750 pounds of superconductor yielding amagnet weighing generally 15,000 pounds (which is light enough to beinstalled in a medical facility) and costing only half or less of what aviable equivalent conventional magnet would cost.

Typically one or more thermal shields are spaced apart from, andsurround, the superconductive main and shielding coils. Forcryogenic-cooling, such thermal shields are located outside thecryogenic vessel. It is noted that the magnet 10 moreover includes, asneeded, thermal spacers and coil forms, as is known to the artisan, forproper spacing and support of the magnet components. In an exemplarydesign, as shown in FIG. 4, the magnet 10 also includes a first thermalshield 86, an inner support cylinder 88, and an outer support cylinder90. The first thermal shield 86 is disposed between, and spaced apartfrom, the first cryogenic vessel 56 and the first vacuum enclosure 70.The inner support cylinder 88 is generally coaxially aligned with thefirst axis 16, is disposed longitudinally outward of the first polepiece 24, has a first end secured to the first vacuum enclosure 70, andhas a second end secured to the first thermal shield 86. The outersupport cylinder 90 is generally coaxially aligned with the first axis16, is disposed longitudinally outward of the first pole piece 24, has afirst end secured to the first thermal shield 86, and has a second endsecured to the first cryogenic vessel 56. Preferably, the inner supportcylinder 88 longitudinally extends a distance generally equal to thedistance that the first thermal shield 86 longitudinally extends outwardfrom the first pole piece 24, and the outer support cylinder 90longitudinally extends a distance generally equal to the distance thatthe first cryogenic vessel 56 longitudinally extends outward from thefirst pole piece 24. In an exemplary construction, first thermal shield86 consists essentially of aluminum, and the inner and outer supportcylinders 88 and 90 consist essentially of a fiber-reinforced compositesuch as carbon fiber or glass fiber. In an example, the above-describedsecuring of the ends of the nonmetallic support cylinders isaccomplished by forming a rounded rim in the ends of the supportcylinders and by using metal rings to capture the rims, some of suchrings being attached to the metallic vacuum enclosure, the metallicthermal shield, or the metallic cryogenic vessel, as appropriate and ascan be appreciated by the artisan. It is noted that the inner and outersupport cylinders 88 and 90 are under tension and provide a superiorsystem for mechanically supporting the magnet components within thefirst vacuum enclosure 70 against the magnetic forces generated by themagnet 10, as can be understood by those skilled in the art. The firstsuperconductive shielding coil 22 has an aluminum overband 92 (withintervening fiberglass insulation) abutting the first cryogenic vessel56 and is wound on a fiberglass coil form 94 supported by discretealuminum brackets 96 (only one of which is shown) attached to the firstcryogenic vessel 56. Discrete aluminum diagonal struts 98 (only one ofwhich is shown) internally brace the first cryogenic vessel 56. Thefirst superconductive main coil 14 has an aluminum overband 100 (withintervening fiberglass insulation) and is wound on a fiberglass coilform 102 which is attached to the first cryogenic vessel 56 and whichhas a flange 104 with helium flow channels 106. There is interveningfiberglass insulation between the first superconductive main coil 14 andthe first cryogenic vessel 56. Discrete aluminum brackets 108 (only oneof which is shown) and a backup ring 110 surround the overband 100 asshown in FIG. 4.

As will be apparent to the artisan in view of the description of theabove embodiments, the magnet of the invention can be more broadlyexpressed as an open magnet 10 having a first assembly 12 and a secondassembly 30 which is longitudinally spaced apart from the first assembly12. The first assembly 12 includes a generally longitudinally-extendingfirst axis 16, at least one main coil 14, at least one shielding coil 22disposed longitudinally outward from the at least one main coil 14, andat least one magnetizable pole piece 24. The at least one main coil 14,shielding coil 22, and magnetizable pole piece 24 are generallycoaxially aligned with the first axis 16. The at least one main coil 14carries a first main electric current in a first direction, and the atleast one shielding coil 22 carries a first shielding electric currentin a direction opposite to the first direction. The at least onemagnetizable pole piece 24 is spaced apart from and proximate the atleast one main and shielding coils 14 and 22. The second assembly 30includes a generally longitudinally-extending second axis 34 generallycoaxially aligned with the first axis 16, at least one main coil 32, atleast one shielding coil 40 disposed longitudinally outward from the atleast one main coil 32, and at least one magnetizable pole piece 42. Theat least one main coil 32, shielding coil 40, and magnetizable polepiece 42 are generally coaxially aligned with the second axis 34. The atleast one main coil 32 carries a second main electric current in thepreviously-described first direction, and the at least one shieldingcoil 40 carries a second shielding electric current in thepreviously-described opposite direction. The at least one magnetizablepole piece 42 is spaced apart from and proximate the at least one mainand shielding coils 32 and 40.

The at least one pole piece enhances the strength of the magnetic fieldso less conductor or superconductor is needed in the at least one maincoil. The portion of the at least one pole piece proximate the at leastone main coil provides a partial magnetic flux return for the at leastone main coil which reduces the magnetizable material needed in the atleast one pole piece and which reduces the amount of conductor orsuperconductor needed in the at least one main coil. The portion of theat least one pole piece proximate the at least one shielding coil alsomagnetically decouples the at least one shielding coil from the at leastone main coil so that the magnetic flux lines from the at least oneshielding coil are captured by the portion of the at least one polepiece proximate the at least one shielding coil and do not reach themagnetic flux lines from the at least one main coil. Therefore themagnetizable mass of the at least one pole piece does not have to beincreased, and the amount of the conductor or superconductor in the atleast one main coil does not have to be increased, to offset the fieldsubtracting effects of the magnetic flux lines from the at least oneshielding coil, since they are blocked by the presence of the portion ofthe at least one pole piece proximate the at least one shielding coil.

Thus, the open magnet 10 is not limited to a superconductive magnet andcan be a resistive magnet or a combination resistive and superconductivemagnet. Likewise, the at least one main and shielding coils 14 & 32 and22 and 40 are not limited to superconductive coils and can be resistiveor a combination of resistive and superconductive coils. It is notedthat the coils 14, 32, 22 and 40 are not limited to being one each, andthe open magnet 10 can have two or more main coils 14, two or more maincoils 32, two or more shielding coils 22, and two or more shieldingcoils 40 or any combination of one or more of such coils. Likewise, eachat least one pole piece 24 and 42 is not limited in number. The shape ofthe coils 14, 32, 22, and 40 is not limited to being generally annular,and the shape of the pole pieces 24 and 42 is not limited to beinggenerally cylindrical, and any shape or combination of shapes suitableto creating a desired magnetic field can be used, as is within thedesign skill of the artisan using the teachings of the presentinvention.

In an example, the open magnet 10 also includes at least one supportmember 48 connecting the first and second assemblies 12 and 30. It isnoted that the at least one support member 48 is not limited in number,and open magnets of the invention can have one, two, three or any numberof support members. It is also noted that the at least one supportmember 48 is not limited to one which does not provide a magnetizablesolid path between the at least one pole piece 42 and the at least onepole piece 24. That is, the at least one pole piece 42 may have nomagnetizable solid path to the at least one pole piece 24, or the atleast one pole piece 42 may have a magnetizable solid path to the atleast one pole piece 24.

In an example, the at least one main coil 14 (whether resistive orsuperconductive) of the first assembly 12 has a cryogenic temperatureduring operation of the magnet 10. It is noted that such cryogenictemperature is required when the at least one main coil 14 comprises andrelies on a superconductor and is permissive but not required when theat least one main coil 14 comprises a conductor (i.e., a resistiveconductor) and does not rely on a superconductor. Likewise, in anotherexample the at least one shielding coil 22 of the first assembly 12 hasa cryogenic temperature during operation of the magnet 10. When acomponent of the first or second assembly 12 or 30 of the magnet 10(such as a coil) has a cryogenic temperature during magnet operation,the component typically is disposed in a corresponding first or secondvacuum enclosure 70 or 76. In the previously-described first coolingoption, the component of the first assembly 12 is disposed in a firstcryogenic vessel 56 which surrounds the component and which is itselfdisposed within the first vacuum enclosure 70. It is noted that, forexample, a component (such as single coil) can have its own vacuumenclosure (or its own cryogenic vessel and vacuum enclosure) or canshare a vacuum enclosure (or a cryogenic vessel and vacuum enclosure)with one or more other components (such as one or more other coils).

From the above description, it will be recognized that the invention canalso be expressed as a method for generating a magnetic field in a firstarea (such as an imaging volume 52) between a first and second assembly12 and 30 while shielding a second area (such as an area longitudinallyand/or radially outward of the assembly pair) not between the first andsecond assembly 12 and 30 from a stray magnetic field emanating from thegeneration of the magnetic field in the first area. In one example, themethod comprises several steps.

One step is disposing as a first assembly 12 at least one main coil 14,at least one shielding coil 22, and at least one magnetizable pole piece24, wherein the at least one main coil 14 carries a first electriccurrent in a first direction, wherein the at least one shielding coil 22is disposed longitudinally outward from the at least one main coil 14and carries a first shielding electric current in a direction oppositeto the first direction, and wherein the at least one magnetizable polepiece 24 is spaced apart from and proximate the at least one main andshielding coils 14 and 22.

Another step is disposing as a second assembly 30 at least one main coil32, at least one shielding coil 40, and at least one magnetizable polepiece 42, wherein the at least one main coil 32 carries a secondelectric current in the previously-described first direction, whereinthe at least one shielding coil 38 is disposed longitudinally outwardfrom the at least one main coil 32 and carries a second shieldingelectric current in the previously-described opposite direction, andwherein the at least one magnetizable pole piece 42 is spaced apart fromand proximate the at least one main and shielding coils 32 and 40. Anadditional step is generally coaxially aligning the at least one mainand shielding coils 14 & 32 and 22 & 40 and pole piece 24 and 42 of thefirst and second assemblies 12 and 30 with a generallylongitudinally-extending axis 16 or 34. A further step is longitudinallyspacing apart the second assembly 30 from the first assembly 12.

The foregoing description of several embodiments and examples of theinvention has been presented for purposes of illustration. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed, and obviously many modifications and variations are possiblein light of the above teaching. It is intended that the scope of theinvention be defined by the claims appended hereto.

What is claimed is:
 1. An open magnet comprising:a) a first assemblyincluding:(1) a longitudinally-extending first axis; (2) at least onemain coil coaxially aligned with said first axis and carrying a firstmain electric current in a first direction; (3) at least one shieldingcoil coaxially aligned with said first axis, disposed longitudinallyoutward from said at least one main coil of said first assembly, andcarrying a first shielding electric current in a direction opposite tosaid first direction; and (4) at least one magnetizable pole piececoaxially aligned with said first axis and spaced apart from said atleast one main and shielding coils of said first assembly, wherein mostof said at least one magnetizable pole piece of said first assembly isdisposed longitudinally between and radially inward of said at least onemain and shielding coils of said first assembly; and b) a secondassembly longitudinally spaced apart from said first assembly andincluding:(1) a longitudinally-extending second axis coaxially alignedwith said first axis; (2) at least one main coil coaxially aligned withsaid second axis and carrying a second main electric current in saidfirst direction; (3) at least one shielding coil coaxially aligned withsaid second axis, disposed longitudinally outward from said at least onemain coil of said second assembly, and carrying a second shieldingelectric current in said opposite direction; and (4) at least onemagnetizable pole piece coaxially aligned with said second axis andspaced apart from said at least one main and shielding coils of saidsecond assembly, wherein most of said at least one magnetizable polepiece of said second assembly is disposed longitudinally between andradially inward of said at least one main and shielding coils of saidsecond assembly.
 2. The magnet of claim 1, also including at least onesupport member connecting said first and second assemblies.
 3. Themagnet of claim 1, wherein said at least one main coil of said firstassembly has a cryogenic temperature during operation of said magnet. 4.The magnet of claim 1, wherein said at least one shielding coil of saidfirst assembly has a cryogenic temperature during operation of saidmagnet.
 5. A method for generating a magnetic field in a first areabetween a first and second assembly while shielding a second area notbetween said first and second assembly from a stray magnetic fieldemanating from the generation of the magnetic field in said first area,the method comprising the following steps:disposing as a first assemblyat least one main coil, at least one shielding coil, and at least onemagnetizable pole piece, wherein said at least one main coil of saidfirst assembly carries a first electric current in a first direction,wherein said at least one shielding coil of said first assembly isdisposed longitudinally outward from said at least one main coil of saidfirst assembly and carries a first shielding electric current in adirection opposite to said first direction, wherein said at least onemagnetizable pole piece of said first assembly is spaced apart from saidat least one main and shielding coils of said first assembly, andwherein most of said at least one magnetizable pole piece of said firstassembly is positioned longitudinally between and radially inward ofsaid at least one main and shielding coils of said first assembly;disposing as a second assembly at least one main coil, at least oneshielding coil, and at least one magnetizable pole piece, wherein saidat least one main coil of said second assembly carries a second electriccurrent in said first direction, wherein said at least one shieldingcoil of said second assembly is disposed longitudinally outward fromsaid at least one main coil of said second assembly and carries a secondshielding electric current in said opposite direction, wherein said atleast one magnetizable pole piece of said second assembly is spacedapart from said at least one main and shielding coils of said secondassembly, and wherein most of said at least one magnetizable pole pieceof said second assembly is positioned longitudinally between said atleast one main and shielding coils of said second assembly; coaxiallyaligning said at least one main and shielding coils and pole piece ofsaid first and second assemblies with a longitudinally-extending axis;and longitudinally spacing apart said second assembly from said firstassembly.